A network facilitates the transmission of messages between its nodes. In a network not all nodes of the network are directly connected to all other nodes. A message from one transmitting node to a receiving node therefore frequently has to be forwarded via one or several intermediate nodes in order to reach the receiving node from the transmitting node. In this case the route from the transmitting node via the intermediate nodes to the receiving node is described as the path or route.
In order to select a suitable path for a message from a large number of theoretically possible paths in a network, a routing method is used. Firstly, the routing method determines at least one, but usefully a plurality of candidate paths along which the message could be transmitted. In the following, a path distance value, a so-called route metric, is always assigned to a candidate path. The path distance value is a measure of the quality of a candidate path. In turn, the path distance value is usually determined from link distance values which, in turn, are a measure of the quality of individual links of the respective candidate path. In this case a link is described as the direct connection between two nodes of the network.
Utilization costs, for example, can be included in the path distance value for one link of the path or the number of links of a path. Furthermore, it is possible for values for a transmission quality along the candidate path or one link of the candidate path or values for the transmission speed of the candidate path or of one link of the candidate path, to be included. The candidate path with the optimal path distance value is subsequently selected as the path. The message can now be transmitted along this path.
The methods for determining the path distance value are termed routing metrics. A known routing metric is ETX (Expected Transmission Count). The path that is selected by the ETX routing metric is the one where the expected number of transmissions is the lowest. Here transmissions are understood to be initial transmissions (transmissions) as well as repeated transmissions (retransmissions). An initial transmission is the transmission of a packet via a link. A repeated transmission occurs if the first transmission was not successful. The initial transmissions and the repeated transmissions are treated equally by ETX.
However, retransmissions have the disadvantage that they can require more time than initial transmissions. Therefore, for certain types of data transmission in specific scenarios, the disadvantage of ETX is that it does not determine the optimal path. Such data transmissions can be voice over IP (VoIP) or video telephony, for example. Other examples of such types of data transmissions are all types of time-critical data transmissions.